Radial foil bearing

ABSTRACT

The radial foil bearing ( 3, 3 A,  3 B) includes: a housing ( 12 ) provided with an insertion hole ( 12   a ), a pair of engagement grooves ( 15 ) extending outward in a radial direction from an inner peripheral edge of the insertion hole being provided in both end surfaces in an axial direction of the housing; a back foil ( 11 ) disposed on an inner circumferential surface of the insertion hole; an intermediate foil ( 10 ) supported by the back foil; and an engagement member ( 30 ) including a pair of engagement legs ( 31 ) engaging with the pair of engagement grooves, and a connection portion ( 32 ) connecting the pair of engagement legs. The intermediate foil is provided with a recess ( 10   c ) recessed and protruding toward the back foil, and the connection portion is disposed in the recess.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is a U.S. national stage application under 35 U.S.C. § 371 of International Patent Application No. PCT/JP2018/009943 filed on Mar. 14, 2018, and which was published in Japanese on Sep. 20, 2018, as International Publication No. WO 2018/168919 A1 under PCT Article 21(2). Priority is claimed on Japanese Patent Application No. 2017-050127, filed Mar. 15, 2017, the content of which is incorporated herein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable.

THE NAMES OF THE PARTIES TO A JOINT RESEARCH AGREEMENT

Not Applicable.

INCORPORATION-BY-REFERENCE OF MATERIAL SUBMITTED ON A COMPACT DISC OR AS A TEXT FILE VIA THE OFFICE ELECTRONIC FILING SYSTEM (EFS-WEB)

Not Applicable.

STATEMENT REGARDING PRIOR DISCLOSURES BY THE INVENTOR OR A JOINT INVENTOR

Not Applicable.

BACKGROUND OF THE INVENTION Technical Field

The present disclosure relates to a radial foil bearing.

Heretofore, as a bearing for a high speed rotating body, a radial bearing is known which is used in a state of encircling a rotary shaft. As such a radial bearing, a radial foil bearing is well known which includes a thin sheet-shaped top foil forming a bearing surface, a back foil elastically supporting the top foil, and a cylindrical bearing housing accommodating the top foil and the back foil. As the back foil of the radial foil bearing, a bump foil obtained by forming a thin sheet into a wave sheet shape is mainly used.

In such a radial foil bearing, an intermediate foil is inserted between the top foil and the back foil for the purpose of “improving the damping effect due to friction between foils”, “increasing the rigidity of the top foil” or the like (refer to, for example, Patent Document 1). The intermediate foil is formed into a thin sheet shape, elastically contacts the tops of hill parts of the wave sheet-shaped bump foil, and causes energy dissipation due to friction through sliding with respect to the tops, thereby damping the film-pressure fluctuation. That is, it is possible to limit axial vibration (self-excited vibration) of the rotary shaft by this damping effect and to easily settle the axial vibration.

DOCUMENT OF RELATED ART Patent Document

[Patent Document 1] Japanese Unexamined Patent Application, First Publication No. 2014-20463

BRIEF SUMMARY OF THE INVENTION Technical Problem

An object of the present disclosure is to improve the damping effect due to the friction between the intermediate foil and the back foil described above.

Solution to Problem

In order to solve the above problems, a radial foil bearing of one aspect of the present disclosure includes: a housing provided with an insertion hole, a pair of engagement grooves extending outward in a radial direction from an inner peripheral edge of the insertion hole being provided in both end surfaces in an axial direction of the housing; a back foil disposed on an inner circumferential surface of the insertion hole; an intermediate foil supported by the back foil; and an engagement member including a pair of engagement legs engaging with the pair of engagement grooves, and a connection portion connecting the pair of engagement legs. In addition, the intermediate foil is provided with a recess recessed and protruding toward the back foil, and the connection portion is disposed in the recess.

In the above aspect of the present disclosure, the back foil may be formed into a wave sheet shape, and the recess may be disposed at a position in a circumferential direction between a top of at least one hill part and a top of a hill part adjacent to the one hill part in the wave sheet shape of the back foil.

In the above aspect of the present disclosure, a side surface of the hill part of the wave sheet shape of the back foil may be a curved surface, and a contact surface of the recess contacting the side surface may be an inclined surface.

In the above aspect of the present disclosure, both ends in the axial direction of each of the intermediate foil and the back foil may be provided with cutouts with which the pair of engagement legs engage.

Effects

According to the present disclosure, it is possible to improve the damping effect due to friction between an intermediate foil and a back foil.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a side view showing an example of a turbo machine to which a radial foil bearing of the present disclosure is applied.

FIG. 2 is a diagram showing a first embodiment of the radial foil bearing of the present disclosure.

FIG. 3A is a development view of a top foil provided in the radial foil bearing shown in FIG. 2.

FIG. 3B is a side view showing, in a flattened manner, the top foil provided in the radial foil bearing shown in FIG. 2.

FIG. 4 is a perspective view showing an important part of the radial foil bearing shown in FIG. 2.

FIG. 5A is a diagram schematically showing, in a flattened manner, the important part of the radial foil bearing shown in FIG. 2.

FIG. 5B is a side view showing, in a flattened manner, the important part of the radial foil bearing shown in FIG. 2.

FIG. 6 is an enlarged view of the important part shown in FIG. 5B.

FIG. 7A is a diagram showing a second embodiment of the radial foil bearing of the present disclosure and schematically showing an important part of the radial foil bearing in a flattened manner.

FIG. 7B is a diagram showing the second embodiment of the radial foil bearing of the present disclosure and is a side view showing the important part of the radial foil bearing in a flattened manner.

FIG. 8 is an enlarged cross-sectional view taken along line A-A in FIG. 7A.

FIG. 9A is a diagram showing a third embodiment of the radial foil bearing of the present disclosure and schematically showing an important part of the radial foil bearing in a flattened manner.

FIG. 9B is a diagram showing the third embodiment of the radial foil bearing of the present disclosure and is a side view showing the important part of the radial foil bearing in a flattened manner.

FIG. 10 is an enlarged cross-sectional view taken along line B-B in FIG. 9A.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, the radial foil bearing of the present disclosure will be described in detail with reference to the drawings.

FIG. 1 is a side view showing an example of a turbo machine to which a radial foil bearing of the present disclosure is applied. The reference sign 1 in FIG. 1 represents a rotary shaft, the reference sign 2 represents an impeller provided at one end in the axial direction of the rotary shaft, and the reference sign 3 represents the radial foil bearing of the present disclosure. Although only one radial foil bearing is shown in FIG. 1 omitting another bearing, in general, two radial foil bearings are provided in the axial direction of the rotary shaft 1 to configure a support structure for the rotary shaft 1. Therefore, two radial foil bearings 3 are also provided in this embodiment.

The radial foil bearing 3 encircles the rotary shaft 1, that is, the rotary shaft 1 is inserted therethrough. A thrust collar 4 is provided on the rotary shaft 1 between the impeller 2 and the radial foil bearing 3. Thrust bearings 5 are disposed on both sides of the thrust collar 4 in the axial direction so as to face the thrust collar 4, that is, the shaft is inserted through the thrust bearings 5.

The impeller 2 is disposed inside a housing 6 that is the stationary portion, and a tip clearance 7 is provided between the impeller 2 and the housing 6.

First Embodiment

FIG. 2 is a diagram showing a first embodiment of the present disclosure. As shown in FIG. 2, a radial foil bearing 3 of the first embodiment is a cylindrical bearing provided with an insertion hole 12 a, encircling and supporting the rotary shaft 1. A direction parallel to the central axis of the radial foil bearing 3 (namely, the central axis of the insertion hole 12 a) is referred to as an axial direction, a direction crossing the central axis is referred to as a radial direction, and a direction around the central axis is referred to as a circumferential direction. The radial foil bearing 3 is configured including a cylindrical top foil 9 (namely, the top foil 9 encircling a circumferential side surface (outer circumferential surface) of the rotary shaft 1) disposed to face the circumferential side surface of the rotary shaft 1, an intermediate foil 10 disposed on radially outside of the top foil 9, a back foil 11 disposed on radially outside of the intermediate foil 10, and a bearing housing 12 (housing) disposed on radially outside of the back foil 11.

The bearing housing 12 is a cylindrical member that configures the outermost part of the radial foil bearing 3. The side surface of the bearing housing 12 is provided with the insertion hole 12 a (a hole causing the bottom surface and the top surface of the cylindrical member to communicate with each other), and the rotary shaft 1 is inserted through the insertion hole 12 a. The back foil 11, the intermediate foil 10, and the top foil 9 are arranged in this order radially inward from outside between the bearing housing 12 and the rotary shaft 1. That is, the back foil 11, the intermediate foil 10, and the top foil 9 are accommodated in the insertion hole 12 a of the bearing housing 12. Thus, the back foil 11 is supported by an inner circumferential surface of the insertion hole 12 a, the intermediate foil 10 is supported by the back foil 11, and the top foil 9 is supported by the intermediate foil 10. The bearing housing 12 may be a member other than a cylindrical member (for example, a square post) as long as the insertion hole 12 a is provided therein. A groove 14 is formed on the inner circumferential surface of the insertion hole 12 a of the bearing housing 12 in the axial direction of the bearing housing 12. That is, the groove 14 is formed on the inner circumferential surface of the bearing housing 12 on the entire length in the axial direction of the bearing housing 12. In other words, in a cross-section of the bearing housing 12 in a direction orthogonal to the axial direction of the insertion hole 12 a, the inner circumferential surface is provided with a recess that is recessed radially outward. The groove 14 can accommodate ends of the top foil 9.

As shown in FIG. 2, a pair of engagement grooves 15 extending radially outward from the inner peripheral edge of the insertion hole 12 a are formed in both side surfaces (end surfaces in the axial direction) of the bearing housing 12. The engagement groove 15 of this embodiment is formed at each of positions by which the side surface of the bearing housing 12 is divided into approximately three areas in the circumferential direction. That is, three pairs of the engagement grooves 15 are provided in the bearing housing 12 of this embodiment. Engagement members 30 described later engage with the engagement grooves 15. In this embodiment, the groove 14 is disposed between two pairs of engagement grooves 15 among the three pairs of the engagement grooves 15. In addition, one pair of engagement grooves 15 faces the groove 14 in the radial direction. In order to form the engagement grooves 15, cutting machining with an end mill, electrolytic machining, wire-cut electric discharge machining or the like can be appropriately used. In addition, the engagement groove 15 may not be formed to penetrate from the inner peripheral edge to the outer peripheral edge of the bearing housing 12. For example, the engagement groove 15 may open only at the inner circumferential surface of the bearing housing 12.

The top foil 9 is cylindrically wound along the inner surface of the intermediate foil 10 and is arranged such that a first uneven portion 23 a formed in one end of the top foil 9 and a second uneven portion 23 b formed in another end thereof engage with the groove 14 formed in the bearing housing 12. As shown in FIG. 3A that is a development view of the top foil 9, the top foil 9 is formed by winding a rectangular metal foil whose long side is in the bearing circumferential direction and whose short side is in the bearing length direction (the axial direction), and the metal foil is cylindrically wound in the arrow direction (the length direction of the long side; the bearing circumferential direction) in FIG. 3B that is a side view of the top foil 9.

As shown in FIG. 3A, the first uneven portion 23 a including one protruding part 21 a and two recessed parts 22 a is formed in one side (short side) of the top foil 9, and the second uneven portion 23 b including two protruding parts 21 b and one recessed part 22 b is formed in another side (short side) thereof opposite to the one side (short side). The recessed part 22 b of the second uneven portion 23 b is formed corresponding to the protruding part 21 a of the first uneven portion 23 a, and the recessed parts 22 a of the first uneven portion 23 a are formed corresponding to the protruding parts 21 b of the second uneven portion 23 b.

That is, the recessed part 22 b of the second uneven portion 23 b is formed such that the protruding part 21 a passes through the recessed part 22 b, when the top foil 9 is cylindrically wound such that the first uneven portion 23 a and the second uneven portion 23 b overlap each other. Similarly, the recessed parts 22 a of the first uneven portion 23 a are formed such that the protruding parts 21 b pass through the recessed parts 22 a, when the top foil 9 is cylindrically wound.

As shown in FIG. 2, the protruding parts 21 a and 21 b passed through the recessed parts 22 b and 22 a, respectively, are pulled out toward the bearing housing 12, and the end parts of the protruding parts 21 a and 21 b are accommodated in the groove 14 of the bearing housing 12 (the ends of the top foil 9 are in contact with and held by inner wall surfaces of the groove 14). That is, both ends in the circumferential direction of the top foil 9 are held by an inner circumferential surface of the insertion hole 12 a of the bearing housing 12. The top foil 9 is arranged such that the ends thereof contact inner surfaces of the groove 14.

As shown in FIG. 3B, the top foil 9 is provided with thin portions 24 at a side (the one side) thereof on which the first uneven portion 23 a is formed and at a side (the other side) thereof on which the second uneven portion 23 b is formed, and the thin portions 24 are thinner than a central portion of the top foil 9 between the sides. As shown in FIG. 2, each of these thin portions 24 is formed to be thin (thinned) such that the outer circumferential surface (the surface close to the bearing housing 12) thereof is recessed compared to the outer circumferential surface of the central portion.

The thin portion 24 is formed to have a desired thickness (thinness) by controlling the thickness of both ends of the top foil 9 in the order of 10 μm through, for example, etching. Specifically, in a case where the bearing diameter is 35 mm, when the thickness of the top foil 9 is 100 μm, the thickness of the thin portion 24 is about 80 μm.

As shown in FIG. 2, the length L in the circumferential direction of the thin portion 24 shown in FIG. 3B is a length corresponding to the groove 13 and one hill part 11 c of an end of the back foil 11. In addition, the length L in the circumferential direction of the thin portion 24 may be a length corresponding to the groove 14 and about three hill parts 11 c of the end of the back foil 11, instead of the example shown in FIG. 2.

As shown in FIG. 2, the back foil 11 is disposed on the inner circumferential surface of the insertion hole 12 a of the bearing housing 12. The back foil 11 is formed of a foil (thin sheet) and elastically supports the intermediate foil 10 and the top foil 9. For such a back foil 11, for example, a bump foil, a spring foil described in Japanese Unexamined Patent Application, First Publication No. 2006-57652, Japanese Unexamined Patent Application, First Publication No. 2004-270904 or the like, or a back foil described in Japanese Unexamined Patent Application, First Publication No. 2009-299748 or the like is used. In this embodiment, a bump foil is used for the back foil 11. However, the spring foil or the back foil described above may be used for the back foil of the present disclosure.

In the present disclosure, the back foil 11 is configured of three (a plurality of) back foil pieces 11 a disposed in the circumferential direction of the bearing housing 12. In each of these back foil pieces 11 a, a foil (thin sheet) is formed into a wave sheet shape in the circumferential direction. In addition, the back foil piece is formed such that the side shape thereof viewed in the axial direction becomes a substantially arc shape as a whole. In the present disclosure, all of the three back foil pieces 11 a are formed to have equal shape and size. Therefore, these back foil pieces 11 a are arranged to divide the inner circumferential surface of the bearing housing 12 into approximately three areas. The number of the back foil pieces configuring the back foil 11 may be appropriately changed.

In addition, the “wave sheet shape” in this embodiment is not limited to a shape configured only of curved surfaces (for example, a sine wave), but denotes a shape in which a radially inward protruding part and a radially outward protruding part are alternately disposed in the circumferential direction. The shape may have a flat portion, namely a linearly extending portion, when viewed in the axial direction and may be configured by combining a plurality of flat portions together.

The back foil pieces 11 a at positions between which the groove 14 is interposed are arranged with a gap. On the other hand, at the other positions, the ends of the back foil pieces 11 a are arranged to be close to each other (with a gap less than the gap in which the groove 14 is positioned). That is, the back foil piece 11 a does not extend to the position in the circumferential direction of the groove 14. By such a configuration, the three back foil pieces 11 a are formed in a substantially cylindrical shape as a whole and are disposed along the inner circumferential surface of the bearing housing 12. That is, when the back foil piece 11 a is viewed in the axial direction, hill parts 11 c protruding radially inward and valley parts 11 b protruding radially outward compared to the hill parts 11 c are alternately formed in the circumferential direction.

The valley part of this embodiment includes the flat valley part 11 b facing the bearing housing 12. The flat valley part 11 b can contact the inner circumferential surface of the insertion hole 12 a. In addition, the hill part 11 c can contact the intermediate foil 10 (an intermediate foil piece 10 a described later). Therefore, the back foil piece 11 a elastically supports the top foil 9 via the intermediate foil piece 10 a, particularly using the hill parts 11 c contacting the intermediate foil piece 10 a (the intermediate foil 10).

In addition, fluid passageways in the radial direction of the radial foil bearing 3 are formed by the hill parts 11 c or the valley parts 11 b. Furthermore, both ends in the circumferential direction of the back foil piece 11 a of this embodiment are the valley parts.

FIG. 5A is a diagram schematically showing an important part of FIG. 2 in a flattened manner. A cutout 16 is formed in each of edges of both ends in the axial direction of a portion (the central portion in a direction parallel to the circumferential direction of the bearing housing 12) of each back foil piece 11 a between both ends thereof in the circumferential direction. That is, a position of the edge extending in the circumferential direction of the back foil piece 11 a, which is between both ends thereof in the circumferential direction, is provided with a recess that is recessed toward the central position thereof in the axial direction. As shown in FIG. 4, the cutout 16 is formed in the valley part 11 b of the back foil piece 11 a. The cutout 16 of this embodiment is formed by cutting out an area including the valley part 11 b and root parts of the hill parts 11 c adjacent to each other across the valley part 11 b, toward the center in the axial direction of the bearing housing 12. That is, the cutout 16 is formed in a position extending in the circumferential direction and including the valley part 11 b. The cutout 16 is formed at a position corresponding to the engagement groove 15 of the bearing housing 12, that is, a position overlapping the engagement groove 15. The width in the circumferential direction of the cutout 16 is formed to be the same as the width in the circumferential direction of the engagement groove 15.

As shown in FIG. 2, the intermediate foil 10 is disposed between the top foil 9 and the back foil 11 configured of the three back foil pieces 11 a. In this embodiment, the intermediate foil 10 is configured of three intermediate foil pieces 10 a arranged in the circumferential direction of the bearing housing 12. As shown in FIG. 5A, the intermediate foil piece 10 a is formed such that the developed shape thereof becomes a substantially rectangular shape, and is curved at a predetermined curvature such that a substantially cylindrical shape is formed of the three intermediate foil pieces 10 a, thereby having an arc shape in side view. Each intermediate foil piece 10 a of this embodiment faces the hill parts 11 c of the back foil piece 11 a in the radial direction.

That is, the intermediate foil piece 10 a can come into contact with the hill parts 11 c disposed at positions in the circumferential direction closest to both ends in the circumferential direction of the back foil piece 11 a. Moreover, the separation distance in the circumferential direction between the back foil pieces 11 a adjacent to each other without the groove 14 disposed therebetween is less than the separation distance in the circumferential direction between the intermediate foils 10 corresponding to the back foil pieces 11 a and positioned on radially inside of the back foil pieces 11 a.

As can be understood from the disclosure to this point, the radial foil bearing 3 of this embodiment includes the top foil 9 formed of one sheet of foil, the back foil 11 formed of three sheets of foil, and the intermediate foil 10 formed of three sheets of foil. The number of the intermediate foil pieces configuring the intermediate foil 10 may be appropriately changed.

As shown in FIGS. 5A and 5B, the intermediate foil piece 10 a includes flat portions 10 b that contact the tops of the hill parts 11 c of the back foil 11, and a recess 10 c (protruding part) that is recessed (protrudes) radially outward compared to the flat portions 10 b.

The recess 10 c is formed so as to be recessed radially outward from the radially inner surface of the intermediate foil piece 10 a and to protrude radially outward from the radially outer surface of the intermediate foil piece 10 a.

That is, the recess 10 c is away from the top foil 9. As shown in FIG. 5A, the recess 10 c is formed at a position in the circumferential direction between both ends in the circumferential direction of the intermediate foil piece 10 a. The recess 10 c of this embodiment includes a bottom positioned radially outward and being flat in the circumferential direction, and tapered parts positioned on both ends in the circumferential direction of the bottom and extending radially inward. That is, the recess 10 c extends radially inward from the bottom as it goes away from the bottom in the circumferential direction. The separation in the circumferential direction between the tapered parts of the recess 10 c gradually decreases radially outward from the radially inner side thereof. In addition, the width in the circumferential direction of the bottom of the recess 10 c is greater than the width in the circumferential direction of the valley part 11 b of the back foil piece 11 a. In this embodiment, the valley part 11 b is formed to be flat. However, if the hill part 11 c and the valley part 11 b have one peaks and are periodically (namely, sinusoidally) formed, then the width in the circumferential direction of the valley part 11 b in the present disclosure is considered to be the dimension of the valley part 11 b of the back foil piece 11 a at the position in the radial direction of the middle between the peak of the hill part 11 c and the peak of the valley part 11 b. The outer shape of the intermediate foil piece 10 a has substantially the same size as the outer shape of the back foil piece 11 a. All of the three intermediate foil pieces 10 a are formed to have equal shape and size. Therefore, these intermediate foil pieces 10 a are disposed to divide the inner circumferential surface of the bearing housing 12 into approximately three areas.

The intermediate foil pieces 10 a are disposed at positions corresponding to the back foil pieces 11 a. The intermediate foil pieces 10 a at positions between which the groove 14 is interposed are disposed with a gap. The intermediate foil pieces 10 a at the other positions are disposed such that the ends thereof are close to each other. That is, in this embodiment, the valley parts 11 b positioned at both ends of the back foil piece 11 a do not reach the position in the circumferential direction of the groove 14. The intermediate foil pieces do also not reach the position in the circumferential direction of the groove 14. The thickness of the intermediate foil piece 10 a is less than that of the back foil piece 11 a. The rigidity of the intermediate foil 10 is less than or equal to half the rigidity of the back foil 11. By such a configuration, the three intermediate foil pieces 10 a are formed in a substantially cylindrical shape as a whole and are disposed to be supported by the back foil 11 along the inner circumferential surface of the bearing housing 12.

As shown in FIG. 5A, a cutout 17 is formed in each of edges of both ends in the axial direction of a portion (the central portion in a direction parallel to the circumferential direction of the bearing housing 12) of each intermediate foil piece 10 a between both ends thereof in the circumferential direction. That is, a position of the edge extending in the circumferential direction of the intermediate foil piece 10 a, which is between both ends thereof in the circumferential direction, is provided with a recess that is recessed toward the central position in the axial direction. As shown in FIG. 4, the cutout 17 is formed in the recess 10 c of the intermediate foil piece 10 a. The cutout 17 of this embodiment is formed by cutting out part of the bottom of the recess 10 c formed between the flat portions 10 b, toward the center in the axial direction of the bearing housing 12 from the side edge.

The cutout 17 is formed at a position corresponding to the engagement groove 15 of the bearing housing 12 and to the cutout 16 of the back foil piece 11 a, namely, a position overlapping the engagement groove 15 and the cutout 16, and the width in the circumferential direction of the cutout 17 is formed to be the same as the width in the circumferential direction of each of the engagement groove 15 and the cutout 16.

An engagement member 30 engages with the engagement groove 15 and the cutouts 16 and 17. As shown in FIGS. 5A and 5B, the engagement member 30 includes a pair of engagement legs 31 extending radially outward and a connection portion 32 extending in the axial direction, and the connection portion 32 connects the pair of engagement legs 31 to each other. One of the engagement legs 31 engages with the engagement groove 15 and the cutouts 16 and 17 on one side of the radial foil bearing 3, and the other of the engagement legs 31 engages with the engagement groove 15 and the cutouts 16 and 17 on the other side of the radial foil bearing 3. That is, the engagement leg 31 is inserted in the engagement groove 15.

As shown in FIG. 5B, the length in the radial direction of the engagement leg 31 is approximately equal to the sum of the thickness of the bearing housing 12, the thickness of the back foil piece 11 a, and the thickness of the intermediate foil piece 10 a. The width of the engagement leg 31 is approximately equal to the width of each of the engagement groove 15 and the cutouts 16 and 17. That is, as shown in FIG. 4, the connection portion 32 is disposed inside the valley part 11 b at the central portion in the circumferential direction of the back foil piece 11 a and inside the recess 10 c of the intermediate foil piece 10 a.

By such a configuration, since the engagement leg 31 engages with the engagement groove 15 of the bearing housing 12, the cutout 16 of the back foil piece 11 a, and the cutout 17 of the intermediate foil piece 10 a, the engagement member 30 serves as a holding member (retainer) that holds the intermediate foil piece 10 a and the back foil piece 11 a on the bearing housing 12. In addition, the connection portion 32 of the engagement member 30 is covered with the top foil 9. In other words, the connection portion 32 is held between the bottom of the recess 10 c of the intermediate foil 10 and the top foil 9.

The engagement leg 31 and the connection portion 32 of the engagement member 30 may be in the shape of a square pole as shown in FIG. 4 or may be in the shape of a column (round bar). The thickness of the engagement member 30 is set such that the connection portion 32 is arranged to be separated from the top foil 9 without contacting the top foil 9. The engagement member 30 can be formed, for example, by etching a metal foil made of stainless steel or the like into a U-shape. It can also be formed by bending a wire-shaped metal rod.

As shown in FIG. 6 that is an enlarged view of an important part shown in FIG. 5B, in a portion in which the engagement member 30 is disposed, the intermediate foil piece 10 a and the back foil piece 11 a can contact each other at positions between which the engagement member 30 is interposed. That is, in the intermediate foil piece 10 a, in addition to a contact point P1 in which the flat portion 10 b contacts the peak of the hill part 11 c of the back foil piece 11 a, a contact point P2 is formed in which the recess 10 c contacts a portion other than the peak of the hill part 11 c of the back foil piece 11 a.

At positions between which a bottom surface 11 b 1 is interposed as shown in FIG. 4, as shown in FIG. 6, the recess 10 c of the intermediate foil piece 10 a contacts side surfaces 11 c 1 of the hill parts 11 c of the back foil piece 11 a by the engagement member 30. The side surfaces 11 c 1 are positioned on both sides in the circumferential direction of the peak of the hill part 11 c and radially outward compared to the peak. That is, the recess 10 c is disposed at a position in the circumferential direction between the peak of one hill part 11 c and the peak of a hill part 11 c adjacent to the one hill part 11 c in the wave sheet shape of the back foil piece 11 a. In other words, the recess 10 c is disposed at a position in the circumferential direction corresponding to a portion of the wave sheet shape of the back foil piece 11 a between the peak of one hill part 11 c and the peak of a hill part 11 c adjacent to the one hill part 11 c. The side surface 11 c 1 of the hill part 11 c is a curved surface, and a side-wall surface 10 c 1 (a contact surface, a radially outer surface of the tapered part) of the recess 10 c in contact with the side surface 11 c 1 is an inclined surface. The separation in the circumferential direction between a pair of side-wall surfaces 10 c 1 gradually decreases radially outward from the radially inner side thereof. That is, the side surface 11 c 1 of the hill part 11 c and the side-wall surface 10 c 1 of the recess 10 c contact each other at one point of the contact point P2 and are separated from each other at other positions, and energy dissipation easily occurs due to friction at the contact point P2. Thus, two contact points P2 are formed in one engagement member 30.

As shown in FIG. 6, the side surface 11 c 1 of the hill part 11 c is a curved surface bulging radially inward when viewed in the axial direction, but may linearly extend when viewed in the axial direction. The side-wall surface 10 c 1 of the recess 10 c linearly extends when viewed in the axial direction, but may be a curved surface bulging radially outward when viewed in the axial direction. If at least one of the side surface 11 c 1 and the side-wall surface 10 c 1 is a curved surface bulging toward the other thereof, these surfaces can contact each other at one contact point and be separated from each other at other positions.

Next, the operation of the radial foil bearing 3 having such a configuration will be described.

In a state where the rotary shaft 1 stops, the top foil 9 is pushed by the back foil 11 (three back foil pieces 11 a) via the intermediate foil 10 (three intermediate foil pieces 10 a) toward the rotary shaft 1 and thus closely contacts the rotary shaft 1. In addition, in this embodiment, since both ends of the top foil 9 are the thin portions 24, at the thin portions 24, a force (local preload) that clamps the rotary shaft 1 does almost not occur.

Then, when the rotary shaft 1 is started in an arrow P direction in FIG. 2, the rotary shaft 1 starts rotating at a low speed at first, and then gradually accelerates to rotate at a high speed. Then, as shown by an arrow Q in FIG. 2, ambient fluid is drawn in from one end of each of the top foil 9, the intermediate foil 10, and the back foil 11, and flows into a space between the top foil 9 and the rotary shaft 1. As a result, a fluid lubrication film is formed between the top foil 9 and the rotary shaft 1.

The film pressure of the fluid lubrication film acts on the top foil 9 and presses each hill part 11 c of the back foil piece 11 a via the intermediate foil 10 in contact with the top foil 9. Then, the back foil piece 11 a is pressed by the intermediate foil 10 so that the hill part 11 c is pressed and expanded, whereby the back foil piece 11 a starts moving in the circumferential direction on the bearing housing 12. That is, since the back foil piece 11 a (the back foil 11) elastically supports the top foil 9 via the intermediate foil 10, the back foil piece 11 a deforms in the circumferential direction when receiving a load from the top foil 9, and accepts flexure of the top foil 9 or the intermediate foil 10 and supports them.

As shown in FIG. 5B, the engagement leg 31 of the engagement member 30 is inserted in and engaged with the cutout 16 provided on the side circumferential edge part of the back foil piece 11 a and serves as a rotation stopper with respect to the bearing housing 12. Thus, each hill part 11 c of the back foil piece 11 a deforms (moves) in the circumferential direction in a state where the cutout 16 with which the engagement member 30 is engaged serves as a fixed point (a fixed end), but the center of the back foil piece 11 a does not move from a fixed position.

The back foil piece 11 a is affected by the friction between the back foil piece 11 a and the bearing housing 12 or the intermediate foil 10 when deforming (moving) in the circumferential direction, and thus easily deforms (moves) at both ends, namely, free end sides thereof, but does not easily deform at the fixed point (the fixed end) side thereof.

Therefore, a difference may occur in the support rigidity of the back foil piece 11 a between the free end side and the fixed end side thereof. However, in this embodiment, since the cutout 16 is formed in the central portion in the circumferential direction of the back foil piece 11 a, the fixed point by the engagement member 30 is in the central portion in the circumferential direction of the back foil piece 11 a, the distance between the fixed end and the free end decreases, and the difference in the support rigidity decreases.

Furthermore, in this embodiment, since the back foil 11 is divided into the three back foil pieces 11 a, the distance between the fixed end and the free end decreases compared to a case where the back foil 11 is configured of a single foil, and thus the difference in the support rigidity between the fixed end side and the free end side thereof further decreases.

Even when an unexpected impact or the like acts while the rotary shaft 1 rotates at a high speed, since the engagement member 30 limits the back foil piece 11 a from moving in the axial direction, the back foil piece 11 a does not drop off the bearing housing 12. Similarly, the intermediate foil piece 10 a is provided with the recess 10 c in which the connection portion 32 of the engagement member 30 is disposed, the engagement leg 31 of the engagement member 30 is disposed in the cutout 17 formed in the recess 10 c, and the intermediate foil piece 10 a is engaged with the engagement groove 15 via the engagement member 30, whereby even when an unexpected impact or the like acts, the intermediate foil piece 10 a does not rotate inside the bearing housing 12, and is limited from moving in the axial direction inside the bearing housing 12. In addition, since the engagement member 30 is covered with the top foil 9 in the radial direction, the engagement member 30 is limited from dropping off the radial foil bearing 3. As a result, the intermediate foil piece 10 a is limited from dropping off the radial foil bearing 3.

The intermediate foil 10 is provided with the recess 10 c in which the connection portion 32 of the engagement member 30 can be disposed, and the recess 10 c protrudes toward the back foil 11 disposed radially outward, whereby as shown in FIG. 6, the recess 10 c contacts the side surfaces 11 c 1 of the hill parts 11 c of the back foil 11 at positions between which the bottom surface 11 b 1 of the valley part 11 b of the back foil 11 is interposed. Therefore, the intermediate foil 10 and the back foil 11 do not contact each other only at the flat portion 10 b but also contact at the recess 10 c, and the contact area therebetween increases, so that the damping effect due to friction between the intermediate foil 10 and the back foil 11 is improved, and the stability when the rotary shaft 1 rotating at a high speed is supported can be further enhanced.

As shown in FIG. 6, two contact points P2 are formed per one engagement member 30, and in this embodiment, as shown in FIG. 2, the two contact points P2 are formed in each of pairs (three pairs) of the intermediate foil piece 10 a and the back foil piece 11 a, so that the contact area between the intermediate foil 10 and the back foil 11 can be increased by six contact points P2 in addition to the contact point P1. In addition, the side surface 11 c 1 of the hill part 11 c of the back foil 11 is a curved surface, the side-wall surface 10 c 1 of the recess 10 c contacting the side surface 11 c 1 is an inclined surface, so that these surfaces contact at one point of the contact point P2 and are separated from each other at other positions. Therefore, the intermediate foil 10 and the back foil 11 do not easily bold each other, and “sliding” therebetween easily occurs.

Second Embodiment

Next, a second embodiment of the radial foil bearing of the present disclosure will be described. In the following description, the same or equal component as or to that of the above-described embodiment is attached with an equal reference sign, and the explanation thereof will be simplified or omitted.

FIGS. 7A and 7B are diagrams showing a radial foil bearing 3A of the second embodiment applied to the turbo machine shown in FIG. 1, FIG. 7A is a diagram schematically showing an important part of the radial foil bearing 3A in a flattened manner, and FIG. 7B is a side view thereof.

The second embodiment is different from the above-described embodiment in that an intermediate foil 10A (an intermediate foil piece 10 a) is provided with branching parts 40 (protruding part) branched off from a flat portion 10 b. That is, a portion protruding radially outward is provided in a radially outer surface of the intermediate foil piece 10 a.

As shown in FIG. 7A, a slit 41 is formed in the flat portion 10 b, and the portion surrounded by the slit 41 is pushed out (cut and raised) so as to protrude radially outward, whereby the branching part 40 is formed. In this embodiment, the slit 41 is formed of two incisions parallel to the circumferential direction, and one incision connecting ends of the two incisions and parallel to the axial direction. The rectangular branching part 40 surrounded by the slit 41 is slantingly pushed out to protrude radially outward. That is, the branching part 40 extends from the ends of the two parallel incisions not connecting to the incision parallel to the axial direction (the ends on far side from the incision parallel to the axial direction).

The branching part 40 separates in the circumferential direction from the above ends not connecting to the incision parallel to the axial direction and separates radially outward.

That is, the branching part 40 extends radially outward as it goes in the circumferential direction toward the incision parallel to the axial direction, from the ends of the two parallel incisions on far side from the incision parallel to the axial direction.

In other words, the position of the branching part 40 monotonously changes radially outward as it separates in the circumferential direction from the ends not connecting to the incision parallel to the axial direction. That is, the branching part 40 of this embodiment lineally extends when viewed in the axial direction. The branching part 40 protrudes toward the back foil piece 11 a at a position in the circumferential direction between the peak of at least one hill part 11 c and the peak of a hill part 11 c adjacent to the one hill part 11 c in the wave sheet shape of the back foil piece 11 a.

As shown in FIG. 7B, a pair of branching parts 40 are provided such that the hill part 11 c of the back foil piece 11 a is interposed therebetween in the circumferential direction. That is, as shown in FIG. 7A, the slit 41 is formed on each of both sides in the circumferential direction of a portion corresponding to the hill part 11 c of the back foil piece 11 a, and a pair of branching parts 40 pushed out from these slits 41 extend away from each other. That is, two pairs of incisions parallel to the circumferential direction are provided to overlap the position in the circumferential direction of one hill part 11 c, and the two pairs are away from each other in the circumferential direction. The separation in the circumferential direction between the pair of branching parts 40 protruding toward one hill part 11 c gradually increases radially outward from the radially inner side thereof. In addition, the incision parallel to the axial direction extend from the ends on the side apart from each other of the two incisions parallel to the circumferential direction. By this configuration, two branching parts 40 are configured such that the hill part is interposed therebetween, both ends thereof in the circumferential direction protrude radially outward, and a portion between the ends is positioned radially inward. As shown in FIG. 7B, two branching parts 40 (a pair of branching parts) provided between which the hill part 11 c of the back foil piece 11 a is interposed in the circumferential direction are in contact with the hill part 11 c of the back foil piece 11 a so as to hold the hill part 11 c from both sides thereof in the circumferential direction. In this embodiment, as shown in FIG. 7A, the flat portion 10 b is provided with the slits 41 in two rows in the circumferential direction, and four branching parts 40 contact one hill part 11 c. In addition, the slits 41 (the branching parts 40) may be provided in a single row or in three or more rows in the circumferential direction.

FIG. 8 shows an enlarged cross-sectional view taken along line A-A of an important part shown in FIG. 7A. The flat portion 10 b of the intermediate foil 10 (a portion positioned on a plane and obtained by tracing an approximately circular closed curve surrounded in the circumferential direction and with no branching, in the axial direction width of the intermediate foil 10) is provided with two branching positions 43 from which the branching parts 40 branches off, between the peak of a first hill part 11 c (the left hill part 11 c shown in FIG. 8) and the peak of a second hill part 11 c (the right hill part 11 c shown in FIG. 8) adjacent to each other in the circumferential direction of the back foil 11. In addition, in this embodiment, the flat portion 10 b is provided with a second branching position 43 (the right branching position 43 shown in FIG. 8) at a position different from a branching position 43 (the left branching position 43 shown in FIG. 8), between the peak of the first hill part 11 c and the peak of the second hill part 11 c. That is, two branching parts 40 are formed between the peak of the first hill part 11 c and the peak of the second hill part 11 c adjacent to each other in the circumferential direction of the back foil 11. In other words, two (a pair of) branching positions 43 are provided in the circumferential direction between the valley parts 11 b adjacent to each other in the circumferential direction. In the radial foil bearing 3 of this embodiment, in a state where the rotary shaft 1 inserted therethrough does not rotate, the intermediate foil piece 10 a and the back foil piece 11 a are separated from each other in the radial direction at a position sifted in the radial direction from the branching position 43.

That is, in the radial foil bearing 3, in a state where the inserted rotary shaft 1 does not rotate, the intermediate foil piece 10 a and the back foil piece 11 a are separated from each other in the radial direction at a position equal in the circumferential direction to the branching position 43.

A pair of branching parts 40, which open in directions away from each other, are in contact with positions (the side surfaces 11 c 1) between which the peak of the hill part 11 c is interposed in the circumferential direction. That is, the intermediate foil piece 10 a is provided with contact points P3 in which the branching parts 40 contact portions other than the peaks of the hill parts 11 c of the back foil piece 11 a, in addition to the contact points P1 in which the flat portion 10 b contacts the peaks of the hill parts 11 c of the back foil piece 11 a.

In addition, in a state where the inserted rotary shaft 1 does not rotate, no contact point may be formed between the two contact points P3 between which the peak of the hill part 11 c. Even in this case, when the inserted rotary shaft 1 rotates, the contact point P1 is formed between the two contact points P3 between which the peak of the hill part 11 c is interposed. In addition, the radially outer surface of the branching part 40 extends from the peak side of a hill part 11 c toward a valley part 11 b adjacent to the hill part 11 c beyond the contact point P3. However, the radially outer surface thereof does not reach the position in the radial direction of the valley part 11 b adjacent to the hill part 11 c.

The side surface 11 c 1 of the hill part 11 c is a curved surface, and the branching part 40 in contact with the side surface 11 c 1 is flat. That is, the side surface 11 c 1 of the hill part 11 c and the branching part 40 contact each other at one point of the contact point P3 and separate from each other at other positions, and thus sliding at the contact point P3 easily occurs.

According to the second embodiment having the above configuration, the branching position 43 and the second branching position 43 at which the branching parts 40 branch off from the flat portion 10 b are provided between the peak of the first hill part 11 c and the peak of the second hill part 11 c adjacent to each other in the circumferential direction of the back foil 11, and as shown in FIG. 8, the branching parts 40 contact positions other than the peak of the hill part 11 c of the back foil 11. Therefore, the intermediate foil 10A and the back foil 11 do not contact each other only at the flat portion 10 b and the recess 10 c but also at the branching part 40 Branched off from the flat portion 10 b, and the number of the contact positions therebetween increases, so that the damping effect due to the friction between the intermediate foil 10A and the back foil 11 is enhanced. That is, two contact points P3 are further formed per one hill part 11 c.

Since a pair of branching parts 40 are provided such that the hill part 11 c of the back foil 11 is interposed therebetween in the circumferential direction, even when the back foil 11 deforms so as to extend or shrink in the circumferential direction, the contact state with respect to the hill part 11 c can always be maintained, and the damping effect due to the friction between the intermediate foil 10A and the back foil 11 can be enhanced.

Third Embodiment

Next, a third embodiment of the radial foil bearing of the present disclosure will be described. In the following description, the same or equal component as or to that of the above embodiments is attached with an equal reference sign, and the explanation thereof will be simplified or omitted.

FIGS. 9A and 9B are diagrams showing a radial foil bearing 3B of the third embodiment applied to the turbo machine shown in FIG. 1, FIG. 9A is a diagram schematically showing an important part of the radial foil bearing 3B in a flattened manner, and FIG. 9B is a side view thereof.

The third embodiment is different from the first and second embodiments in that an intermediate foil 10B (an intermediate foil piece 10 a) is provided with a branching part 40B (a protruding part) branched off from a flat portion 10 b.

As shown in FIG. 9A, the branching part 40B is formed by forming a slit 41B in the flat portion 10 b and pushing out the portion surrounded by the slit 41B so as to protrude radially outward. That is, the branching part 40B of this embodiment protrudes radially outward from the flat portion 10 b in the circumferential direction and further extends radially inward beyond the peak of protruding. In this embodiment, the slit 41B is formed in an H-shape, and two branching parts 40B formed by the slit 41B bulge in a curved shape so as to protrude radially outward, thereby forming a wave shape. That is, when viewed in the radial direction (refer to FIG. 9A), the branching parts 40B extend from the sides at both ends in the circumferential direction of a rectangular area in which the H-shaped slit 41B is disposed, toward the sides at the ends opposite thereto. The tips of the two branching parts 40B separates from each other in the circumferential direction.

As shown in FIG. 9B, the branching part 40B is disposed at a position corresponding to the valley part 11 b of the back foil piece 11 a. That is, the branching part 40B includes a position facing the valley part 11 b in the radial direction and extends to each of the hill parts 11 c adjacent to both sides of the valley part 11 b in the circumferential direction. However, the tip of the branching part 40B is positioned to be closer to the valley part 11 b than the peak of the hill part 11 c. Therefore, the branching part 40B extends from one hill part 11 c to a position in the circumferential direction of a hill part 11 c adjacent to the one hill part 11 c. As shown in FIG. 9A, the central portion of the slit 41B has an area in which the back foil 11 is exposed when viewed in a radial direction. Through the exposing area, the position corresponding to the peak of the hill part 11 c of the back foil piece 11 a is exposed. The pair of branching parts 40B cut and raised from the slit 41B are deformed in curved surfaces so as to separate from each other, and as shown in FIG. 9B, the branching parts 40B contact hill parts 11 c disposed on both sides of a hill part 11 c disposed at a position corresponding to the center of the slit 41B. That is, a hill part 11 c held from both sides thereof in the circumferential direction by the branching parts 40B and a hill part 11 c not held from both sides thereof in the circumferential direction by the branching parts 40B are alternately arranged in the circumferential direction. In other words, the peak of the hill part 11 c of the back foil piece 11 a and the peak of the branching part 40B alternately appear in the circumferential direction. In further other words, peaks of one branching part 40B are provided between the top of one hill part 11 c and the tops of hill parts 11 c adjacent to the one hill part 11 c. In addition, the “peak” described above denotes a peak of a radial direction position, namely, a portion that protrudes radially outward.

As shown in FIG. 10 that is an enlarged cross-sectional view taken along line B-B of an important part shown in FIG. 9A, the flat portion 10 b is provided with a branching position 43B from which the branching part 40B branches off, between the top of a first hill part 11 c and the top of a second hill part 11 c adjacent to each other in the circumferential direction of the back foil 11. That is, one branching part 40B is formed between the top of the first hill part 11 c and the top of the second hill part 11 c adjacent to each other in the circumferential direction of the back foil 11. The branching part 40B includes a separating part 40B1 that separates radially outward from the flat portion 10 b, an approaching part 40B2 that extends from the separating part 40B1 (the radially outer end of the separating part 40B1) and approaches radially inward and toward the flat portion 10 b, and an end part 40B3 that extends from the approaching part 40B2 (the radially inner end of the approaching part 40B2) along the flat portion 10 b. The separating part 40B1 and the approaching part 40B2 have smoothly curved shapes with an equal radius of curvature. That is, the separating part 40B1 and the approaching part 40B2 are formed in a curved surface bulging radially outward as a whole. In addition, each of the separating part 40B1 and the approaching part 40B2 may be configured by combining a linear shape and another linear shape or by combining a linear shape and a curved shape, when viewed in the axial direction.

The separating part 40B1 of the branching part 40B is in contact with the side surface 11 c 1 of the hill part 11 c. That is, the intermediate foil piece 10 a is provided with a contact point P4 at which the separating part 40B1 of the branching part 40B contacts a portion other than the top of the hill part 11 c of the back foil piece 11 a, in addition to the contact point P1 at which the flat portion 10 b contacts the top of the hill part 11 c of the back foil piece 11 a. The side surface 11 c 1 of the hill part 11 c is a curved surface, and the separating part 40B1 of the branching part 40B in contact with the side surface 11 c 1 is a curved surface that is convex toward the side surface 11 c 1. That is, the side surface 11 c 1 of the hill part 11 c and the separating part 40B1 of the branching part 40B contact each other at one point of the contact point P4 and separate from each other at other positions, and thus sliding at the contact point P4 easily occurs. In other words, in this embodiment, contact points P4 are formed by two branching parts 40B between which the top of one hill part 11 c is interposed in the circumferential direction. On the other hand, a hill part 11 c adjacent to the above hill part 11 c has no contact point with respect to the branching part 40B. In addition, the length from the contact point P4 to the tip of the branching part 40B is greater than the length between the branching position 43B of the branching part 40B and the contact point P4. That is, in this embodiment, the length from the contact point P4 to the tip of the branching part 40B is greater than the length to the contact point P4 from the top of a hill part 11P interposed between the contact points P4.

According to the third embodiment having the above configuration, the branching position 43B at which the branching part 40B branches off from the flat portion 10 b is provided between the top of the first hill part 11 c and the top of the second hill part 11 c adjacent to each other in the circumferential direction of the back foil 11, and as shown in FIG. 10, the branching part 40B is in contact with a position other than the top of the hill part 11 c of the back foil 11. Therefore, the intermediate foil 10B and the back foil 11 do not contact each other only at the flat portion 10 b and the recess 10 c but also at the branching part 40B branched off from the flat portion 10 b, and the number of the contact positions therebetween increases, so that the damping effect due to the friction between the intermediate foil 10B and the back foil 11 is enhanced.

Since a pair of branching parts 40B are provided such that every other hill part 11 c is interposed, even when the back foil 11 deforms so as to extend or shrink in the circumferential direction, the branching parts 40B can contact the hill part 11 c. In addition, the branching part 40B has a wave sheet shape including the separating part 40B1 that separates radially outward from the flat portion 10 b, and the approaching part 40B2 that extends from the separating part 40B1 and approaches radially inward and toward the flat portion 10 b, and when the back foil 11 deforms in the circumferential direction, the branching part 40B is pushed by the hill part 11 c and causes sliding (namely, extends in the circumferential direction) as shown by a reference sign S in FIG. 10, similar to the back foil 11 (a bump foil).

At this time, “sliding” can also occur between the end part 40B3 of the branching part 40B and the top foil 9, and the damping effect due to friction is further enhanced.

Hereinbefore, although the embodiments of the present disclosure have been described with reference to the attached drawings, the present disclosure is not limited to the above embodiments. The shapes, combinations, and the like of the components described in the above embodiments are merely examples, and addition, omission, replacement, and other modifications of the configuration can be adopted based on design requirements and the like within the scope of the present disclosure.

For example, the configuration and number of grooves and retainers for holding the top foil 9, the intermediate foil 10, and the back foil 11 are not limited to the above embodiments, and various configurations may be adopted therefor.

Although the bearing housing 12 is formed in a cylindrical shape in the above embodiments, one side surface or both side surfaces of the bearing housing may be provided integrally with an annular flange, thereby forming the housing in an approximately cylindrical shape as a whole. By forming the flange, the bearing can be easily attached to a housing or the like of a turbo machine. 

The invention claimed is:
 1. A radial foil bearing, comprising: a housing provided with an insertion hole, a pair of engagement grooves extending outward in a radial direction from an inner peripheral edge of the insertion hole being provided in both end surfaces in an axial direction of the housing; a back foil disposed on an inner circumferential surface of the insertion hole; an intermediate foil supported by the back foil; and an engagement member including a pair of engagement legs engaging with the pair of engagement grooves, and a connection portion connecting the pair of engagement legs, wherein the intermediate foil is provided with a recess recessed and protruding toward the back foil, and the connection portion is disposed in the recess.
 2. The radial foil bearing according to claim 1, wherein the back foil is formed into a wave sheet shape, and the recess is disposed at a position in a circumferential direction between a top of at least one hill part and a top of a hill part adjacent to the one hill part in the wave sheet shape of the back foil.
 3. The radial foil bearing according to claim 2, wherein a side surface of the hill part of the wave sheet shape of the back foil is a curved surface, and a contact surface of the recess contacting the side surface is an inclined surface.
 4. The radial foil bearing according to any one of claims 1 to 3, wherein both ends in the axial direction of each of the intermediate foil and the back foil are provided with cutouts with which the pair of engagement legs engage. 